scholarly journals When spectroscopy fails: The measurement of ion pairing

2006 ◽  
Vol 78 (8) ◽  
pp. 1571-1586 ◽  
Author(s):  
Glenn Hefter
Keyword(s):  
Metal Ion ◽  
Chemical Speciation ◽  
Ion Pairs ◽  
Ion Association ◽  
Ion Pair ◽  
Association Constants ◽  
Spectroscopic Techniques ◽  
Association Equilibria ◽  
Solvent Molecules ◽  
Contact Ion Pairs

Spectroscopic techniques such as UV/vis, NMR, and Raman are powerful tools for the investigation of chemical speciation in solution. However, it is not widely recognized that such techniques do not always provide reliable information about ion association equilibria. Specifically, spectroscopic measurements do not in general produce thermodynamically meaningful association constants for non-contact ion pairs, where the ions are separated by one or more solvent molecules. Such systems can only be properly quantified by techniques such as dielectric or ultrasonic relaxation, which can detect all ion-pair types (or equilibria), or by traditional thermodynamic methods, which detect the overall level of association. Various types of quantitative data are presented for metal ion/sulfate systems in aqueous solution that demonstrate the inadequacy of the major spectroscopic techniques for the investigation of systems that involve solvent-separated ion pairs. The implications for ion association equilibria in general are briefly discussed.

Octahedral cobalt(III) complexes in dipolar aprotic solvents. II. Association between cis- and trans-dichlorobisethylenediamine cobalt(III) cations,[Co en2 Cl2]+, and chloride and bromide ions in the solvents NN-dimethylformamide, dimethyl sulphoxide, and LVN-dimethylacetamide

1966 ◽  
Vol 19 (1) ◽  
pp. 43 ◽  
Author(s):  
WA Millen ◽  
DW Watts
Keyword(s):  
Spectrophotometric Method ◽  
Ion Pairs ◽  
Ion Association ◽  
Ion Pair ◽  
Association Constants ◽  
Dimethyl Sulphoxide ◽  
Aprotic Solvents ◽  
Ion Association Constants ◽  
Cis And Trans ◽  
Dipolar Aprotic Solvents

Ion association constants at 30� have been determined for the cis-[Co en, Cl2]+Cl- ion pair in NN-dimethylformamide (DMF), NN-dimethylacetamide (DMA), and at 20.0�, 25.0�, and 30.0� in dimethyl sulphoxide (DMSO), by a spectrophotometric method. Association constants for the cis-[Co en2 Cl2]+Br- and the trans- [Co en2 Cl2]+Cl- ion pairs have also been determined in DMF at 30�.

Effects of substituents on ion-association properties. II. 1H N.M.R. studies of substituted pyridinium and tetraalkylammonium Bis(4-methylbenzene-1,2-dithiolato)cobaltate(III) in non-aqueous solvents

1981 ◽  
Vol 34 (11) ◽  
pp. 2321 ◽  
Author(s):  
N Tsao ◽  
Y Lim
Keyword(s):  
Dielectric Constant ◽  
Complex Anion ◽  
Ion Pairs ◽  
Ion Association ◽  
Ion Pair ◽  
Association Constants ◽  
Ionic Clusters ◽  
Isotropic Shift ◽  
Large Size ◽  
Pyridinium Cations

A series of nine compounds containing quaternary ammonium and substituted pyridinium cations and the bis(4-methylbenzene-1,2-dithiolato)cobaltate(III) complex anion have been prepared andtheir ion-pair properties studied by lH n.m.r. spectroscopy. Their concentration association constants in nitrobenzene at 307 K range from 9 to 20. In the pyridinium series of ion pairs, it is concludedfrom the measured isotropic shift ratios that methyl substituents at the meta and para positions favour a geometry where the anion is tilted towards the ring of the cation but the ortho substituents push the anion away from the ring. In the methyltrioctylammonium ion pair, there is linear correlation between the observed isotropic shift of the N-methyl or the N-methylene protons and the dielectric constant of the solvents (E) from 4.56 to 12.3 for the 0.06, 0.09 and 0.12 M solutions. Its implication is discussed in terms of the formation of the ionic clusters of sufficiently large size.

Ion-ion-solvent interactions in solution. Aqueous solutions of nitrates of cations from Groups 2A and 3A

1982 ◽  
Vol 35 (9) ◽  
pp. 1793 ◽  
Author(s):  
DW James ◽  
RL Frost
Keyword(s):  
Aqueous Solutions ◽  
Ion Pairs ◽  
Ion Pair ◽  
Band Shape ◽  
Nitrate Ion ◽  
Solvent Interactions ◽  
Contact Ion Pair ◽  
Component Band ◽  
Association Equilibria ◽  
Contact Ion Pairs

Association equilibria in aqueous solutions of the salts Be(NO3)2, Ca(NO3)2 Sr(NO3)2, Ba(NO3)2 and Al(NO3)3 have been studied through band shape and component band analysis of the non-degenerate, Raman-active V1 vibrational band of the nitrate ion. Band contributions due to the aquated nitrate ion, solvent-separated ion pair, and contact ion pair were made for all salts. The nature of the contact ion pair was shown to be different in the presence of Be2+ and Al3+ from that seen for the other salts. This difference was associated with a strongly directional perturbation of the nitrate ion. For all salts the solvent-separated ion pair species was present in the lowest concentration solution studied (as low as 0.05 M). At intermediate concentrations the solvent separated ion pair species was the dominant solution species while at the higher concentrations the species with anion and cation in contact becomes appreciable. For solutions of Ca(NO3)2 some evidence for a more extensive ion aggregate was found at the highest concentration (5M). Equilibrium quotients calculated from the spectroscopic components are in reasonable agreement with previous values determined by equilibrium and transport measurements, and it is suggested that these previous determinations detected the presence of solvent-separated ion pairs rather than contact ion pairs.

Ion-ion-solvent interactions in solution. I. Solutions of LiClO4 in acetone

1982 ◽  
Vol 35 (9) ◽  
pp. 1775 ◽  
Author(s):  
DW James ◽  
RE Mayes
Keyword(s):  
Raman Band ◽  
Ion Pairs ◽  
Ion Association ◽  
Solvent Interactions ◽  
Solvation Numbers ◽  
Solvated Ions ◽  
Component Band ◽  
Li Ion ◽  
Contact Ion Pairs ◽  
Association Equilibrium

Vibrational spectra and 7Li, 13C and 35Cl n.m.r. spectra have been obtained for solutions of LiClO4 in acetone for salt concentrations from 0.05 to 6 M. Infrared spectra give qualitative indications of ion association. Analysis of the Raman band due to C-C stretching in acetone yields solvation numbers for the Li+ ion of the order of 3. Component band analysis of the ClO4- symmetric stretching vibrational band and the various n.m.r. spectra lead to the identification of solvent-separated ion pairs, contact ion pairs and ion aggregates, in addition to free solvated ions. The dependence on salt concentration of all four species has been determined. The association quotient for the association equilibrium (Li+)s(ClO4)- ↔ [Li+(acetone)ClO4-)s was determined to be 1.4 � 0.3 dm3 mol-1.

Synthesis, Structure, and Ion Pair Dynamics of β-Diketiminato-Supported Organoscandium Contact Ion Pairs

2005 ◽  
Vol 24 (6) ◽  
pp. 1173-1183 ◽  
Author(s):  
Paul G. Hayes ◽  
Warren E. Piers ◽  
Masood Parvez
Keyword(s):  
Ion Pairs ◽  
Ion Pair ◽  
Contact Ion Pairs

Picosecond Dynamics of Molecular Entities in Lithium Salt Solutions in Dimethyl Sulfoxide, Propylene Carbonate, and Dimethyl Carbonate

2018 ◽  
Vol 63 (3) ◽  
pp. 245 ◽  
Author(s):  
M. I. Gorobets ◽  
S. A. Kirillov
Keyword(s):  
Dimethyl Sulfoxide ◽  
Propylene Carbonate ◽  
Molten Salts ◽  
Dimethyl Carbonate ◽  
Lithium Salt ◽  
Ion Pairs ◽  
Lithium Salts ◽  
Strong Interactions ◽  
Solvent Molecules ◽  
Contact Ion Pairs

An analysis of the Raman spectra of the solutions of lithium salts in dimethyl sulfoxide, propylene carbonate, and dimethyl carbonate in a concentration range from diluted solutions to the mixtures of molten solvates with salts has been performed in terms of the dynamics, specifically, the dephasing (тv) and modulation (тw) times of all molecular entities present in solutions are determined and analyzed. It has been found that, in the picosecond time domain, the dephasing and modulation in solvent molecules hydrogen-bonded with an anion and/or solvating a cation are slower than in free solvent molecules. In solvent separated ion pairs, both тv and тw are much longer than in solvated anions, thus indicating the strong interactions between anions and their surrounding. In contact ion pairs, тv are great, whereas тw appear close to those for free anions. This reflects that the structure of the liquid tends to the structure of molten salts.

A Raman spectral study of solvation and ion association in the systems LiAsF6/CH3CO2CH3 and LiAsF6/HCO2CH3

1991 ◽  
Vol 69 (11) ◽  
pp. 1766-1773 ◽  
Author(s):  
Zhongyi Deng ◽  
Donald E. Irish
Keyword(s):  
Methyl Acetate ◽  
Methyl Formate ◽  
Ion Pairs ◽  
Lithium Ion ◽  
Ion Association ◽  
Ion Solvation ◽  
Lithium Cation ◽  
Different Types ◽  
Raman Spectral ◽  
Contact Ion Pairs

The structure of the solvated lithium cation in methyl acetate (MA) solutions has been investigated using Raman spectroscopy. Two bands at 844 and 864 cm−1 have been assigned to two different types of MA: the former is from the bulk solvent and the latter arises from MA molecules solvating the lithium cation. From measurement of changes in intensity of these bands with increasing salt concentration a solvation number of four for Li+ in MA has been inferred. Changes in the Raman bands at ca. 1740 cm−1 suggest that solvation occurs through the carbonyl group. Evidence for contact ion pairing between Li+ and AsF6− is also presented. An equilibrium between solvent-shared ion pairs and contact ion pairs is proposed for which an equilibrium constant is estimated. The system LiAsF6/methyl formate (MF) is similar in structure. Key words: Raman, ion pair formation, lithium and hexafluoroarsenate ions, methyl acetate and formate, lithium ion solvation.

scholarly journals Aqueous Contact Ion Pairs of Phosphate Groups with Na+, Ca2+ and Mg2+ – Structural Discrimination by Femtosecond Infrared Spectroscopy and Molecular Dynamics Simulations

2020 ◽  
Vol 234 (7-9) ◽  
pp. 1453-1474 ◽  
Author(s):  
Benjamin P. Fingerhut ◽  
Jakob Schauss ◽  
Achintya Kundu ◽  
Thomas Elsaesser
Keyword(s):  
Ion Pairs ◽  
Model System ◽  
Ion Pair ◽  
Pair Formation ◽  
Phosphate Group ◽  
Phosphate Ion ◽  
2D Ir ◽  
Dna And Rna ◽  
Contact Ion Pairs ◽  
Phosphate Groups

AbstractThe extent of contact and solvent shared ion pairs of phosphate groups with Na+, Ca2+ and Mg2+ ions in aqueous environment and their relevance for the stability of polyanionic DNA and RNA structures is highly debated. Employing the asymmetric phosphate stretching vibration of dimethyl phosphate (DMP), a model system of the sugar-phosphate backbone of DNA and RNA, we present linear infrared, femtosecond infrared pump-probe and absorptive 2D-IR spectra that report on contact ion pair formation via the presence of blue shifted spectral signatures. Compared to the linear infrared spectra, the nonlinear spectra reveal contact ion pairs with increased sensitivity because the spectra accentuate differences in peak frequency, transition dipole moment strength, and excited state lifetime. The experimental results are corroborated by long time scale MD simulations, benchmarked by density functional simulations on phosphate-ion-water clusters. The microscopic interpretation reveals subtle structural differences of ion pairs formed by the phosphate group and the ions Na+, Ca2+ and Mg2+. Intricate properties of the solvation shell around the phosphate group and the ion are essential to explain the experimental observations. The present work addresses a challenging to probe topic with the help of a model system and establishes new experimental data of contact ion pair formation, thereby underlining the potential of nonlinear 2D-IR spectroscopy as an analytical probe of phosphate-ion interactions in complex biological systems.

scholarly journals Spectrophotometric and theoretical studies on the determination of etilefrine hydrochloride in pharmaceutical formulations and biological samples

2017 ◽  
Vol 16 (10) ◽  
pp. 2487-2500
Author(s):  
Ahmed Mohamed El Defrawy ◽  
Amr Lotfy Saber
Keyword(s):  
Methyl Orange ◽  
Correlation Coefficient ◽  
Density Functional ◽  
Ion Pairs ◽  
Ion Association ◽  
Pharmaceutical Formulations ◽  
Ion Pair ◽  
Bromocresol Green ◽  
Geometrical Parameters

Purpose: To develop a simple and cost effective spectrophotometric method for the determination of etilefrine hydrochloride (ET) in pharmaceutical formulations and human plasma.Methods: The method is based on extraction of ET into chloroform as ion-pair  complexes with bromocresol green (BCG) and methyl orange (MO) in acidic medium. The interaction of ET with BCG and MO reagents were investigated using  B3LYP/6-31G(d) level of theory. The geometrical parameters of the interacting species and the ion pairs formed were characterized based on their frontier  molecular orbitals, atomic charges, electrostatic potential map, as well as NBO analysis.Results: The colored species exhibited absorption maxima at 410 and 479 nm for the two systems in universal buffer of pH range (3.0 - 3.5), with molar absorptivity of 2.4 × 104 and 1.7 × 104 Lmol-1cm-1, for BCG and MO methods, respectively. The methods demonstrated good linearity with correlation coefficient ranging from  0.9987 – 0.9991 in the concentration ranges 0.5 – 16 and 2.0 – 18 μgmL-1 for BCG and MO methods, respectively. The composition ratio of the ion-association complexes was 1:1 in all cases as established by Job’s method. Sandell,s  sensitivity, correlation coefficient, detection and quantification limits were also calculated. Molecular descriptors were obtained based on optimized structures of the molecules under investigation, by applying the B3LYP/6-31G(d) method, and used to interpret the mode of interaction between these molecules to form the investigated ion pairs.Conclusion: The proposed methods make use of simple reagents, which a basic  analytical laboratory can afford. No interference was observed from common  pharmaceutical excipients and additives. ETMO ion pair has a larger interaction energy (higher stability) than ET-BCG ion pair as inferred from their interaction energies.Keywords: Density functional theory, Etilefrine hydrochloride, Ion pair complex, Spectrophotometry, Bromocresol green, Methyl orange, Geometric analysis

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